This goal of the project is to design, construct, evaluate and refine the performance of a novel, in-situ carbonate-ion sensor. The sensor will be based on a well-documented chemical reaction with lead (Pb(II)) ions that yield a carbonate salt that absorbs UV light. To develop an in situ instrument using this chemistry, the team must build a housing that can withstand the rigors of deployment at depth, containing pumps, a light source, an optical cell, reagents and storage vessels, and control electronics. The team has already designed and deployed an in situ pH sensor based on similar principles and has a reputation for successful delivery of instrumentation. As concern about ocean acidification grows both within and outside of NOAA, frequent and widespread measurements of carbon cycle parameters become increasingly important. The concentration of carbonate ions is usually calculated from other carbon parameters such as pH, total alkalinity and total dissolved inorganic carbon. The calculations are, however, not perfect and direct measurements would be preferable, particularly in regions such as coral reefs where calcification and calcite and aragonite dissolution occur. Delivery of a robust and relatively inexpensive in situ carbonate ion sensor is a highly desirable outcome for NOAA.